JP-A-2003-69012 corresponding to U.S. Pat. No. 6,764,963 discloses the following with respect to SiC semiconductor devices using an a-face where the plane direction of a crystal face has a (11-20)-surface orientation for a channel: the channel mobility with MOS structure can be enhanced by carrying out hydrogen annealing or treatment in a wet atmosphere. Specifically, the channel mobility is enhanced by selecting a concentration or temperature for hydrogen annealing or a wet atmosphere. The present inventors verified that the channel mobility can be spectacularly enhanced by taking the following measure: an (11-20)-orientation face, i.e., a-face, is subjected to wet oxidation, and the temperature is lowered to 600° C. in the wet atmosphere to terminate defect in an oxide film/Sic interface by hydrogen. (Refer to Japanese Patent Application publication No. 2007-96263 corresponding to US Patent Application No. 2007-0045631-A1.)
It is known that Basal Plane Dislocation (BPD) typified by stacking fault is prone to occur in an (11-20)-orientation face. For this reason, a problem of the passage of a leakage current arising from BPD occurs when a power device, such as MOSFET and PN diode, is formed over a (11-20)-orientation face.
To cope with this, a power device was formed over a (11-20)-orientation face and the causes of leakage suspected to be caused by BPD were investigated. As the result, it was found that the following took place when ion implantation was carried out: even though activation heat treatment is carried out at 1500° C. or higher for curing a crystal defect and activating ion species, BPD cannot be reduced and on the contrary, it is increased and this increases leakage.
An example will be taken. In vertical power MOSFET using a planar channel of SiC, a p-well layer is formed by taking the following measure: p type dopant, such as Al or B, is ion implanted into the surface part of an n− type or n+ type wafer formed by laminating an n− type layer over an n+ type substrate using a mask open in a desired position. For this reason, leakage arising from BPD is caused at a PN junction by activation heat treatment after the ion implantation.
Thus, it is required to suppress leakage at a PN junction having a function of holding high voltage in a power device formed over a (11-20)-orientation face and thus suppress decline in the functions of the power device due to leakage.